Then I decided to make it a CAN Bus sniffer:

https://forums.raspberrypi.com/viewtopic.php?t=141052&hilit=mcp2551

This is also an interesting read in the same direction:

https://karambasecurity.com/blog/2018-01-17-how-to-build-a-can-sniffer

However, I realized I can't test this by itself so I started googling more and found the most amazing piece of open source hardware and software I have seen in the CAN Bus world:

https://github.com/nopnop2002/Arduino-CANBus-Monitor

So I quickly put this together, flashed it, and it works! It's not gonna be mega fast and it will likely miss packages but this is much better than anything I have seen so far. Usually CAN Bus sniffers with interpreter software are quite pricy...